Cisapride is a new prokinetic agent that facilitates or restores motility through the gastrointestinal tract and is widely used to treat gastroesophageal reflux and other disorders of gastrointestinal motility in children. Recently, cisapride has been associated with prolongation of corrected QT interval on the electrocardiogram, complex ventricular arrhythmias (torsade de pointes) and life-threatening events in a few isolated patients (1-3). In addition, a 15 year old girl with the congenital long QT syndrome, well controlled with propranolol and ventricular pacing, but who had sustained severe hypoxic encephalopathic injury prior to the diagnosis of the long QT syndrome, expired unexpectedly several months after beginning cisapride therapy. Because of this reported association between cisapride and lengthening of the corrected QT interval along with life-threatening arrhythmias, we propose to examine the effect of cisapride on the corrected QT interval in young patients taking this medication. We intend to obtain electrocardiograms (ECG) in approximately 35 young subjects before, during, and, if possible and clinically permissible, after cisapride administration. The QT interval in all 12 leads at the 3 sampling intervals will be recorded, measured, corrected for the heart rate by Bazett's formula, and compared using the paired t test. The significance of this study is the possibe detection of potentially adverse effects of cisapride on ventricular repolarization measured on the ECG that could be lead to life-threatening arrhythmias. The detection of such an adverse effect of cisparide on the corrected QT interval (i.e. prolongation) could impact both the clinical use of this widely employed medication and possibly contribute to further investigation into the mechanisms of ventricular repolarization and ventricular arhythmias.